1. Field of the Invention
The invention relates to a wrist watch having a thermoelectric generator provided with thermoelectric elements comprising a plurality of thermocouples which are mounted in a watch case together with a movement for driving hands and serve as a power source for the movement.
2. Description of the Related Art
Each thermoelectric element is a generating element comprising a plurality of thermocouples for converting thermal energy into electric energy, which generates a voltage when there is a difference in temperature supplied to both end faces thereof. A wrist watch which comprises the thermoelectric elements is a wrist watch having a thermoelectric generator. The thermoelectric elements are mounted in a watch case together with a movement for driving the hands and this serves as a power source for the movement. The thermoelectric elements convert heat energy, caused by the difference in temperatures supplied to case back and the metal case body constituting the watch case to electric energy. And the movement is driven by this generated power (thermoelectromotive force).
The internal construction of the conventional wrist watch having a thermoelectric generator is described with reference to FIG. 10 which shows a sectional view thereof and FIG. 11 which shows a rear view thereof with the removal of the case back. FIG. 10 is a sectional view taken along the line C--C of FIG. 11.
A wrist watch having a thermoelectric generator 100 constitutes a watch case comprising an insulating body 120 fixedly attached to the lower side (underside) of the metal case body 110 with a glass plate 111 circular in a plane shape, fixedly attached thereto, a case back 130 made of metal securely fixed to the underside of the insulating body 120. The watch case houses therein a dial 40, hands (hour hand, minute hand) 51 and a movement 50 including a step motor, a gear train, a crystal oscillation circuit for driving the movement and thermoelectric elements 160.
As shown in FIGS. 10 and 11, the thermoelectric elements 160 are mounted inside the insulating body 120 but outside the dial 40 and movement 50. The thermoelectric elements 160 are housed in the watch case in a manner that they are brought into contact with the metal case body 110 at one end face and with the case back 130 at the other end face. The insulating body 120 is provided for insulating between the metal case body 110 and the case back 130 so as to easily bring about the difference in temperatures therebetween.
If a user wears the wrist watch having a thermoelectric generator 100 having such a construction on user's wrist, the case back 130 contacting the wrist is heated by the user's body temperature while the metal case body 110 opposite to the case back 130 is cooled by outside air temperature. Accordingly, the difference in temperatures between the body temperature and the air temperature is supplied to both end faces of the thermoelectric elements 160, and this difference in temperatures generates thermoelectromotive force. The movement 50 is driven by this thermoelectromotive force.
FIG. 12 is a schematic perspective view of the thermoelectric elements 160 to be mounted in this wrist watch.
Each of the thermoelectric elements 160 is of such a size to be mounted in this wrist watch having a thermoelectric generator 100 and comprise a plurality of columnar p-type thermoelectric semiconductor devices 180, a plurality of columnar n-type thermoelectric semiconductor devices 181 which are arranged regularly respecticely, and insulating resins 182 which fill between the p-type thermoelectric semiconductor devices 180 and the n-type thermoelectric semiconductor devices 181, so as to fix them integrally with one another as a whole.
In FIG. 12, respective one faces of the p-type thermoelectric semiconductor devices 180 and n-type thermoelectric semiconductor devices 181 are visible. The p-type thermoelectric semiconductor devices 180 and n-type thermoelectric semiconductor devices 181 are made of a bismuthtelluride (BiTe) alloy semiconductor.
Although not shown, electrode films are provided at both end faces of the respective p-type thermoelectric semiconductor devices 180 and the n-type thermoelectric semiconductor devices 181 for connecting mutually adjoined semiconductors with one another so as to connect these semiconductors in a series as a whole. A pair of thermocouples are constituted by the adjoined p-type thermoelectric semiconductor devices 180 and n-type thermoelectric semiconductor devices 181 which generate thermoelectromotive force respectively in accordance with the difference in temperatures. Since respective thermocouples are connected in a series with each other by the electrode films, the entire thermoelectric elements 160 generate thermoelectromotive force in accordance with the number of thermocouples contained therein.
The thermoelectric elements 160 are mounted in the wrist watch having a thermoelectric generator 100 in a manner that one of the upper and lower end faces thermally contact the metal case body 110 while the other of the upper and lower end faces thermally contact the case back 130 so as to supply a difference in temperature to each pair of thermocouples.
Meanwhile, it is known that the magnitude (power) of the thermoelectric elements 160 is determined by a gross sectional area of the p-type thermoelectric semiconductor devices 180 and n-type thermoelectric semiconductor devices 181 each constituting the thermoelectric elements 160 and the magnitude of the difference in temperatures supplied to both end faces thereof.
Suppose that each length of the p-type thermoelectric semiconductor devices 180 and n-type thermoelectric semiconductor devices 181 are about 2 mm, the difference between both temperatures produced in the wrist watch having a thermoelectric generator 100 is about 1.degree. C. in average.
Considering the material characteristics of the BiTe thermoelectric semiconductor, and since the thermoelectromotive voltage is 200 .mu.V/K, and the specific resistance is about 1.2.times.10.sup.-5 .OMEGA.m including a wiring resistance, the maximum output per unit sectional area is 40 .mu.W/cm.sup.2.
However, this value of the output is applied to the case when all the thermoelectric elements are made up by thermoelectric semiconductors. It must be considered that the p-type semiconductor devices 180 and the n-type semiconductor devices 181 are actually filled with the insulating resins 182 which do not contribute to the generation of power. Suppose that the ratio of the areas occupied by the end faces of the p-type semiconductor devices 180, and n-type semiconductor devices 181 to the insulating resins 182 are 1:1, the areas of the p-type semiconductor devices 180 and n-type semiconductor devices 181 become merely a quarter of the entire area of the thermoelectric elements 160. Accordingly, the maximum output set forth above becomes a quarter of 40 .mu.W/cm.sup.2, namely, 10 .mu.W/cm.sup.2.
On the other hand, although the power to be consumed by the present wrist watch is about 1 .mu.W, the power needed by the wrist watch including power for charging the wrist watch so as to drive the wrist watch even if it is not worn by a user is ten times as much as 1 .mu.W, namely, about 10 .mu.W.
Accordingly, the power of about 10 .mu.W needed by the thermoelectric elements used by the wrist watch having a thermoelectric generator 100 requires about 1 cm.sup.2 as the areas of end faces of the thermoelectric elements. If the thermoelectric elements having the area of end faces of 1 cm.sup.2 are mounted inside the wrist watch, the following problem occurs.
In the case of a wrist watch as shown in FIGS. 10 and 11, suppose that the thermoelectric elements have areas of about 1 cm.sup.2, for example, if the thermoelectric elements each having 1 cm.times.1 cm in length to width are to be housed in the wrist watch, it would be very difficult in view of the available space of the wrist watch because the movement is situated at a central position.
To solve this problem, it is conceived to change the outer shapes of thermoelectric elements so as to be easily housed in a wrist watch. However, in view of the method of manufacture and construction of the thermoelectric elements, the thermoelectric elements are generally manufactured in a rectangular shape as shown in FIG. 12, it is very difficult to change the rectangular shape to a rounded shape with curved surfaces or curved lines. Even if the thermoelectric elements are manufactured in such a shape, there is a possibility that gaps would be produced between the thermoelectric elements and the watch case when they are housed in the watch case, which would cause a problem of the deterioration of filling efficiency of the thermoelectric semiconductor.
Accordingly, conventionally there has been employed a method of mounting the thermoelectric elements in a watch case, as shown in FIGS. 10 and 11, comprising steps of dividing the rectangular thermoelectric elements into several pieces so that the entire area thereof becomes 1 cm.sup.2 and dispersing and disposing each piece of the thermoelectric elements around the movement 50.
In the example, as shown in FIGS. 10 and 11, the thermoelectric elements are divided into four pieces, i.e., thermoelectric elements 160, and they are respectively dispersed and disposed at four positions along the inner periphery of the metal case body 110 outside the movement 50 and dial 40.
However, since most wrist watches are substantially circular in a plane shape as shown in FIG. 11, square thermoelectric elements 160 are not fitted to be mounted in the wrist watches. If the thermoelectric elements are mounted in the wrist watch, there occurs a problem of deteriorating the utilizing efficiency of the internal space of the wrist watch, resulting in a large-sized wrist watch as a whole.
Further, when thermoelectric elements are mounted in a wrist watch having a thermoelectric generator, it is necessary to satisfy the following three mounting conditions because of the utilization of the thermoelectromotive force. That is, (1) one end face of the thermoelectric elements is directly brought into contact with a metal case body, (2) the other end face thereof is brought into contact with the case back, and (3) the metal case body and the case back are thermally insulated from each other.
Accordingly, in the case of the wrist watch having a thermoelectric generator 100 as shown in FIGS. 10 and 11, and since the thermoelectric elements 160 are brought into contact with the metal case body 110 at the other end face thereof while they are brought into contact with the case back 130 at one end face thereof, it is necessary to secure the space for housing the thermoelectric elements 160 by the metal case body 110 outside the dial 40 and dispose the insulating body 120 outside the metal case body 110.
As a result, the wrist watch having a thermoelectric generator 100 has a large area extending from the outer periphery of the dial 40 to the outer periphery of the metal case body 110 so that the entire size of the watch case including the metal case body 110 becomes large compared with the size of the dial 40. Accordingly, there occurs a problem that the balance in size between the dial and the watch case becomes worse as a whole.
As mentioned above, since the area occupied by the thermoelectric elements is determined to obtain a power needed for driving the movement by the thermoelectric elements, it is necessary to secure a space for housing the thermoelectric elements in the watch case to some larger extent. Accordingly, if the wrist watch is to be miniaturized, the dial 40 has to be miniaturized. This causes a serious problem in the manufacture of lady's small-sized wrist watches, namely, this means that lady's wrist watches having a thermoelectric generator are substantially hardly merchandised.
This is described in detail using typical numerical values. In an ordinary wrist watch, the diameter of a dial is about 25 mm. If the thermoelectric element 160 is divided into 4 as shown in FIG. 11 when it is disposed around the dial, each of the thermoelectric elements is of a size, e.g. about 8 mm.times.3.1 mm. Then, the circular annular portion, namely, the metal case body 110 in which the thermoelectric elements are mounted needs a width of about 5 mm. Suppose that the insulating body 120 provided around the circular annular portion has a width of about 5 mm, the diameter of the wrist watch adding these numerical values become approximately bout 45 mm which is too large even for men's wrist watches.
As one of means for solving the problem in a housing space when the thermoelectric elements are mounted in the wrist watch, it is conceived to divide the thermoelectric elements into more pieces, such as 10 or 20 pieces and the divided 10 or 20 pieces are disposed, but this is not actually practical considering the increase of a load which is produced in manufacturing or mounting the thermoelectric elements.
It is conceived, as another means for solving the problem, that a desired thermoelectromotive force is obtained even if each size of the thermoelectric elements is made small, namely, the thermoelectromotive force of the thermoelectric elements per unit volume (thermoelectromotive force density) is made large. To this end, the difference between two temperatures to be supplied to both end faces of the thermoelectric elements may be made large.
Under the circumstances, it is preferable to radiate heat from the metal case body or to absorb the body's temperature by the case back with high efficiency. Then, corresponding thereto, the difference in temperatures occurred between the case back and the metal case body becomes large, resulting in a large thermoelectromotive force density, and hence a sufficient thermoelectromotive force can be obtained even if the thermoelectric elements have small areas.
Accordingly, the size of the metal case body may be made large to enhance the heat radiation from the metal case body, which is however contrary to the miniaturization of the wrist watch, so that the miniaturization of the wrist watch having a thermoelectric generator can not be realized.